A couple weeks or so ago I ran into Gorman Cook, a member of the Adler’s Sheppard Society and an avid high power rocketry fan. As we chatted about Far Horizons he mentioned ThunderStruck, an upcoming launch event he intended to attend. ThunderStruck, now in its fourth year is a production of the Indiana Rocket club, centered in Purdue. It’s a big event and has a big launch site, featuring an altitude limit of 18,000 feet. That’s the highest limit east of the Mississippi! I have to admit, it got me thinking… Ever since I got my Level 1 certification I’ve been thinking about going for my Level 2. After all, we’ll need Level 2 certification for SPARK. But the launch was 10 days off. Could I get everything together in time?
The first step was to get a rocket. We’ve had good luck dealing with Public Missiles Limited in Michigan so that was my first stop. After a bit of browsing I found their “Tethys” kit, a simple but robust rocket that looked approachable in the time allowed. The Tethys is 53″ long and comes with adaptors so it can be flown on 29, 35 and 54mm motors. For my certification flight I’d be wanting to use a J engine which comes in both 35 and 54mm versions so this gave me maximum flexibility. The Tethys also comes with a 36″ parachute. More on that later. Ordering on Friday was easy and the kit arrived on Tuesday morning. Michigan really is just a hop, skip and jump away.
Construction was pretty much straightforward. The new lab is really just an ideal place for doing this sort of work. All the tools at hand and plenty of space. First task was to make sure all the parts fit and that nothing was binding. The nose cone was a bit tight so I filed down one of the fitting rings. Half an hour later the nose cone was sliding smoothly with just a bit of a tug for the final separation. This is just what is wanted. The rule of thumb is that you ought to be able to hold the rocket up by its nosecone without it slipping, but if you give it a few shakes it ought to fall out. Any tighter and it might not eject and deploy the parachute. Any looser and it might slip off during aerodynamic deceleration after engine burnout.
Next step was to assemble the motor mount. The motor mount is essentially to hold the motor straight and firmly, and to allow quick changes of the motor. Usually it consists of a central “phenolic” tube (in this case 54mm in diameter) that holds the motor and a set of spacing rings to center the tube in the rocket body. The adhesive of choice for high power rocketry is always epoxy. I chose a 15 minute setting epoxy to give me a bit of working time, but also to not make me wait too long between bonds.
After the motor mount was in place the next step was to attach the fiberglass fins. Like most high powered rockets, the Tethys has “through the wall” fins. The fins slide through slots in the body tube and butt up against the motor tube with epoxy. To give extra strength the fins are then filleted on the outside of the body tube. As an extra step I filled the space between the two centering rings, and the rocket and body tube with an expanding foaming epoxy that is designed to take the heat of the rocket engine firing. This provided a very solid bond for the fins and the motor tube. They weren’t going anywhere!
Simply sliding the motor (in its casing) into the motor tube wouldn’t be sufficient. While the friction fit would keep the motor from moving around in normal transport, when the motor is ignited it would provide vastly more thrust and slide around. In particular, the motor tube is not sealed at the forward end. The motors after all come in a variety of lengths and also the ejection charge gases need to expand out and deploy the parachute. What this means though is that unless the rocket motor is well restrained the thrust will push the motor forward and out the front of the motor tube into the rest of the rocket. With flames shooting out the back into the inside of your rocket and pushing through the parachute and nosecone… The result is a complete flaming catastrophe! Even if this doesn’t happen then the ejection charge can result in a backward thrust, throwing the motor out the rear of the rocket to plummet to the ground… Also a bad situation.
The motor retention system is the solution to this scenario. Basically this is simply a combination of a design specification for motors and a simple screw-on ring. The specification stipulates that the motor casings all have a “lip” which is wider than the motor mount tube. This prevents the motor from advancing forward into the rocket body further. The screw-on ring then prevents the motor from falling out the back. It is all made of solid aluminum alloy and is probably the sturdiest thing in the rocket.
Attached to the motor tube assembly is the piston, shock cord and parachute. The piston is a closed tube that slides up and down inside the body tube of the rocket. Its job is to protect the parachute from the hot gases of the ejection charge. It is attached at the bottom to the motor tube and from the top to the parachute and nosecone.When the rocket finishes its burn it, after a delay, sets off an ejection charge. The expanding hot gases push the piston upward which in turn pushes the parachute and nosecone out of the body-tube. You can see the piston in the third image in this post.
With the last touch on the piston the rocket was ready to fly! Or was it? For a rocket to be stable the center of pressure (essentially where the drag acts) has to be sufficiently behind the center of gravity. Usually at least 1 body-tube diameter. The center of pressure I calculated with the help of a nifty program I wrote that allows one to estimate the center of pressure depending on the parameters of the rocket. One can also simply read it off the instruction booklet that comes with the kit. The center of gravity is a bit more difficult because while I could measure the center of gravity of the finished rocket, that didn’t include the weight of the rocket motor. And I couldn’t get the rocket motor until I was at the launch site since I didn’t have my level 2 certification yet! The center of gravity without the motor was only just a bit more than a body-tube diameter in front of the center of pressure. It probably would have been ok, but I decided to give myself a bit of insurance. To move the center of gravity up a bit I filled the nose cone with foaming epoxy and used a heavy metal eyebolt to attach the parachute shock cord to the nosecone.
All this took until Friday evening. In my next post I’ll tell what happened at the launch!